To Import a Boat you DO NOT have to give US Customs 72 hours notice... There is no need to EXPORT as the vehicle is not a Land powered vehicle..
About the VIN I have imported a few boats and trailer and sometimes they run out and check the VIN and sometimes not... It would be best to have a Replacement VIN sent out in case they check It could hold up the buyer...
The cost to Import is only the GST (which he would have to pay anyway) and the $210 RIV Fee for the Canadian Tire Inspection...
Check out
www.riv.ca and that site has all the info needed...